Many electrical devices and systems operate at least partly on electrical power stored in storage elements. Storage elements are conventionally manufactured to supply direct-current power at a predetermined voltage level. If the electrical device or system to be powered requires a higher voltage level, the higher voltage level 's obtained by connecting a plurality of storage elements in series.
Currently, the most prevalent type of storage element is a battery. A battery is an electrochemical cell that can be charged electrically to store electric power and provide electric power when needed at a later time. The present invention is of particular significance when applied to two or more series connected batteries, and that application of the present invention will be described in detail below. The present invention may, however, be applied to other types of storage elements. For example, the present invention may be applied to series-connected electrostatic devices such as capacitors or super-capacitors. The term “battery” will thus be used herein to refer to any type of rechargeable storage element that is susceptible to unbalanced charge when connected in series.
Rechargeable batteries are often used to provide power in many situations. Examples of typical situations where rechargeable batteries are used include vehicles and portable computers designed to operate independent of utility power and uninterruptible power supplies that are configured to provide standby power should utility power or other main source of power be interrupted.
A rechargeable battery typically operates in either a charge mode in which the battery stores electrical power or in a supply mode in which the battery forms a source of electrical power. In the charge mode, an external power source is connected across the battery such that current flows into and charges the battery.
When batteries are connected in series, the simplest method of charging the batteries is to connect the external power source to the positive terminal of the first battery in the string and to the negative terminal of the last battery in the string. Ideally, current will flow through the batteries in the string and charge each of the batteries equally.
In practice, however, variations in individual batteries and charging conditions may cause an imbalance in the charge on individual batteries in the string. Continued imbalance of one battery in the string adversely impacts the life of that battery. In addition, when one battery fails, other batteries in the string tend to fail. The capacity of the battery string is limited by the weakest battery in the string, and it has long been recognized that correction of an imbalanced battery string is desirable.
The present invention is particularly suited for use as part of an uninterruptible power supply for use in communications systems such as CATV or telephony systems, and that application will be described herein in detail. It should be understood that the present invention may have broader application to other environments, such as computers or electric vehicles, and the scope of the present invention need not be limited to a particular embodiment designed for communication systems.